Here is my list for the best 10 episodes of South Park ever made (or at least that I have ever seen). Generally I find the funniest episodes to be the ones revolving around Cartman, who for my money is the funniest cartoon character on television.
1) Cartman the Hall Monitor - Cartman gets appointed hall monitor and turns into Dog the Bounty hunter. It's priceless. "Think you can get away without having a hall pass? Think again!" It is similar to the episode where Cartman gets deputized and becomes a cop. Then rides around on his tricycle like an episode of Cops (that ep almost made the list, but other than Cartman becoming a cop, the rest of that episode sucked). "Authority?"
2) Dances with Smurfs - The spoof of Glenn Beck was priceless to say the least, and they tied it in to a ridicule of Avatar. They killed two birds with one episode. The title "Dances With Smurfs" is entirely a ridicule of Avatar, not related to Cartman doing the morning announcements and turning into the Glenn Beck Show.
3) Faith +1 - The episode where Cartman starts his own Christian rock band is absolutely hysterical. This is one that you need to watch to appreciate, as no recap would do it justice.
4) AWESOM-O - Cartman pretends to be a robot, and gets stuck having to play to role longer than anticipated. This is a must see, as I suppose all on this list are too.
5) The Return of the Fellowship of the Ring to the Two Towers - Butters sees a porno that's supposed to be Lord of the Rings and turns into Gollum, while the boys are on a quest to return the movie to the video store. It is righteous.
6) Fingerbang - The kids start a boy band, and if you loathe boy bands, you will love this episode. Perhaps the funniest part of the whole ep is that they name their band "Fingerbang".
7) Tsst - Cartman vs "the dog whisperer" is hilarious. With his mom frustrated with her troubled child, and Nanny 9/11 failing miserably, she calls in Caesar Milan, who basically deals with Cartman like he's the family pet.
8) The Passion of the Jew (the Mel Gibson ep) - The greatest part of this episode is not the boys ripping Mel for being a racist nut, but that they made this episode shortly before the whole Malibu "Sugar Tits" arrest.
9) All about the Mormons - Nobody has a handle on the Mormon community quite like the creators of South Park. They lay out this pyramid scheme of a religion beautifully.
10) Night of the living homeless - This did not become hilarious until the end when they attempt to lure all the homeless people away Pied Piper style with a rendition of Dre and Tupac's California Love. "In the city, city of Santa Monica, lots of rich people giving change to the homeless."